This invention relates to a direct liquid injection system which provides for on-line cleaning, and more particularly to a system and process for the controlled deposition of metal oxide layers using chemical vapor deposition techniques.
Semiconductor devices such as dynamic random access memories (DRAMs) have undergone substantial decreases in size and increases in charge storage density over the past several years, and it is expected that these trends will continue into the future. In order to increase capacity while decreasing size, DRAM designs have become increasingly complex. One problem has been the design of capacitors in the DRAM which will hold the necessary electrical charge representing stored data.
Oxides of silicon have conventionally been used as the dielectric materials in such DRAM capacitors. Silicon oxides, however, have relatively low dielectric constants and limited charge storage densities. Accordingly, there has been an effort in the art to identify materials having higher dielectric constants which are suitable for use in DRAM designs. Interest in ferroelectric materials such as barium strontium titanates (Ba.sub.1-x Sr.sub.x TiO.sub.3, known as BSTs) and lead zirconate titanates (Pb(Zr.sub.x Ti.sub.1-x)O.sub.3, known as PZTs) has grown because such materials have relatively higher dielectric constants than silicon oxides, are structurally stable, and can be prepared using known techniques.
Of the many chemical and physical deposition techniques used in the art to form thin film layers of such ferroelectric materials, metal organic chemical vapor deposition (MOCVD) using direct liquid injection appears to hold the most promise. For example, Desu et al, U.S. Pat. Nos. 5,431,958 and 5,527,567 teach MOCVD techniques, including direct liquid injection ('567 patent), to provided layered ferroelectric films for the manufacture of capacitors. Si et al, U.S. Pat. No. 5,629,229, also teach the manufacture of DRAMs using MOCVD techniques.
In MOCVD, the metalorganic precursors which are used are dissolved in liquid solvents which are then pumped in precise proportions to a vaporizer. The vaporized precursors are then sent to a CVD chamber where they are deposited on a substrate. The composition and properties of the deposited films of the ferroelectric materials are highly dependent on the ability of the direct liquid injection system to supply the correct proportions of precursors to the vaporizer and thence to the CVD chamber. Other variables in the system will also affect the composition and properties of the deposited films and include the condition of the vaporizer, the temperature of the vaporizing surfaces in the vaporizer, the concentrations of the metalorganic precursors in the solvent, and local temperature variations on the substrate surface in the reactor. The local temperature variations as well as accumulation of precursor compounds on the surfaces of the vaporizer both affect vaporization efficiency and can cause fluctuations in the composition and properties of the films which are deposited.
Such accumulations of precursor compounds and oxidative reaction products of the precursor compounds have the tendency to build-up over time and clog both the outlet to the vaporizer as well as the direct liquid injection mechanism. This leads not only to undesirable variations in the ratios of the precursor compounds which are deposited, but also to possible clogging and shutdown of the deposition process. To address these problems, Gardiner et al, U.S. Pat. No. 5,362,328, teach the use of a cleaning subsystem in a chemical vapor deposition process in which a solvent is supplied to the vaporizer to solubilize any deposited compounds and flush them away. However, the Gardiner et al cleaning subsystem requires that the CVD process be periodically shut down during the cleaning cycle. Accordingly, the need still exists in this art for a direct liquid injection system which provides for on-line cleaning without the need for shutting down the CVD process.